In persons with diabetes, sugar is not accumulated in the muscles or the liver due to a deficiency of insulin secreted from the liver or an unresponsiveness of body cells to insulin. Therefore, glucose concentration in the blood, that is, blood sugar level is increased, thereby causing various complications such as retinopathy, neural disorder, nephropathy, and the like. Since the number of diabetes patients in Japan is 6.9 million and the number including patients with pre-diabetes group is more than 13 million, diabetes has become a serious national disease. In the present treatment for diabetes, a complete curing method has not been provided yet. For that reason, blood sugar level is maintained at an adequate level by an insulin injection or dietary treatment while measuring blood sugar levels.
At present, blood sugar level is measured by the use of a measuring instrument utilizing a glucose sensing method which electrochemically analyzes a glucose oxidase reaction for the taken blood to convert the analyzed value into the blood sugar level. A portable measuring instrument for measuring blood sugar level is used for daily management of the blood sugar level of a diabetes patient has already been commercialized. In such a blood sugar level test, there have been problems such as pain accompanied with blood drawing several times a day, infection caused by the needle, and the like. Accordingly, it has been required to have an instrument for measuring the blood sugar level noninvasively which does not require blood drawing and can measure a change in the blood sugar level in a day in real-time.
There has been disclosed a technique for irradiating the human body with light of a wavelength in a near-infrared region, measuring diffusely reflected light or transmitted light from the human body by using a spectroscope, and calculating the blood sugar level of the human body on the basis of the spectrum of the diffusely reflected light or the transmitted light (for example, refer to Non-Patent Documents 1 and 2 and Patent Document 1). Non-Patent Document 1 proposes a method of irradiating the light of a wavelength in the near-infrared region to the skin of the forearm and a standard reflecting plate by turns, measuring the spectrum of each diffusely reflected light by using the spectroscope and the like on the basis of the each diffusely reflected light, and measuring the blood sugar level by a multivariate analysis on the basis of a diffusion reflectance spectrum determined by the ratio of the spectrum of the diffusely reflected light of the skin of the forearm and the standard reflecting plate. Patent Document 1 proposes a method of irradiating the light of wavelength in the near-infrared region on a finger or the like, detecting the transmitted light thereof, evaluating absorbances for the specific wavelengths of 944 nm and 964 nm, and measuring the blood sugar level on the basis of the absorbance values.
However, according to the Non-Patent Document 1, a complicated spectroscope equipped with a diffraction grating and the like has been needed in order to irradiate the light of the wavelength in the near-infrared region on the skin of the forearm and measure the consecutive spectrum of the diffusely reflected light. That is, reflectance data of the light of a consecutive wavelength is needed so as to calculate the blood sugar level, and the above-mentioned spectroscope is needed to obtain the reflection spectrum after irradiating the human body with the light from a white light source having the light of wavelength in such a region. Since each diffusely reflected light from the standard reflecting plate and the human body is measured by turns, a change in light source causes a measurement error. In such a method of measuring the blood sugar level based on such white light source or the spectroscope, it was difficult to miniaturize and portablize the measuring instrument of the blood sugar level which can be carried easily for the diabetes patients to manage their daily blood sugar level.
On the other hand, in Patent Document 1, there has been proposed an instrument for measuring the blood sugar level by the use of light of the two specific wavelengths and the transmitted light thereof. This technique is described with reference to FIG. 9. The measuring instrument shown in FIG. 9 is equipped with a light source 100 generating near-infrared light, a diffraction grating 340, and a reflecting mirror 360 for irradiating a finger 1 only with the predetermined monochromatic light from the near-infrared light; and a sampling prism 370, an ND filter 390, and a photodetector 380 for detecting a part of the dispersed monochromatic light 101. In addition, the measuring instrument is equipped with a lens 50 and a photodetector 51 for detecting transmitted light 102 from the human finger 1; a signal processing section 230 which amplifies detection signals from the photodetectors 51 and 380 to digitize the signals; and a central control section 200. The central control section 200 calculates a transmittance T of the finger 1 on the basis of the detection signals from the photodetectors 51 and 380 amplified and digitized in the signal processing section 230 by the following formula.T=I1/I0  (1.1)
Wherein, I0 is the irradiated light quantity of the irradiated light 101 and calculated by multiplying the detection signal detected by the photodetector 380 by a constant value. In addition, I1 s light quantity of the transmitted light 102 and calculated by multiplying the detection signal detected by the photodetector 51 by a constant value. Here, the two wavelengths of 944 nm and 964 nm are selected as the wavelength of the irradiated light 101, and the transmittances for each wavelength are represented as T1 and T2. Then, the blood sugar level C is calculated by the following formula.C=k0+k1*ABS1/ABS2  (1.2)
Wherein, ABS1 and ABS2 can be obtained by the formulae: ABS1=−ln(T1) and ABS2=−ln(T2), respectively, k0 and k1 represent coefficients determined by a least squares method by the use of the actually measured blood sugar level. Here, the white light source is used as the light source. However, when semiconductor lasers of 944 nm and 964 nm are used as the two different wavelengths, there can be achieved the instrument for noninvasively measuring the blood sugar level which does not need a complicated spectroscope equipped with the diffraction grating and the like.
However, in this prior invention, a linear distance r1 between an irradiated position P0 of the irradiated light 101 and a detected position P1 of the transmitted light 102 is slightly changed depending on the size of the finger 1. Due to the amount of such a slight change, there has been a problem that a significant measurement error occurs in the calculation of the blood sugar level C by the aforementioned formula. Further, even the irradiated position P0 is disposed in the same side of the detected position P1 at a distance of the linear distance r1 so as not to be affected by the size of the finger, there is still a problem in that a significant measurement error occurs in the calculation of the blood sugar level C by the aforementioned formula (1.2) because the transmittance represented as the formula (1.1) changes due to the expansion or contraction of a blood vessel changing in response to the heart rate.
(Non-Patent Document 1)
Katsuhiko Maruko, et. al., IEEE Journal of Selected Optics in Quantum Electronics, Vol. 9, No. 2, pp. 322 to 330, 2003
(Non-Patent Document 2)
H. M. Heise et. al., Artificial Organs, 18(6) pp439 to 447, 1994
(Patent Document 1)
Japanese Unexamined Patent Application Publication No. 5-176917